Answers Favorited

If you don't need and in app store to purchase books i highly recommend GoodReader. Easy sync (through wifi/web/email/itunes and customizable). I used it before iBooks came along (which i now use almost exclusively).

Instapaper is excellent for reading articles you've found on the web, while Kindle is better suited to actual books. In that sense, they have completely different use cases depending on what type of material you're reading. They're both outstanding on iPad and iPhone. As far as I know, Readability on iOS only exists as a webapp at this point, not an app through the App Store.

I have to second GoodReader too. It's easily the most powerful, flexible document reader out there, but, again, it performs almost a completely different task from what either Instapaper or Kindle can do.

A cool tool can be any book, gadget, software, video, map, hardware, material, or website that is tried and true. All reviews on this site are written by readers who have actually used the tool and others like it. Items can be either old or new as long as they are wonderful. We post things we like and ignore the rest. Suggestions for tools much better than what is recommended here are always wanted.